Solar cell with reduced absorber thickness and reduced back surface recombination

ABSTRACT

Manufacture for an improved stacked-layered thin film solar cell. Solar cell has reduced absorber thickness and an improved back contact for Copper Indium Gallium Selenide solar cells. The back contact provides improved reflectance particularly for infrared wavelengths while still maintaining ohmic contact to the semiconductor absorber. This reflectance is achieved by producing a back contact having a highly reflecting metal separated from an absorbing layer with a dielectric layer.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.13/650,494, filed Oct. 12, 2012, titled “Solar Cell With ReducedAbsorber Thickness And Reduced Back Surface Recombination”, which ishereby incorporated by reference.

BACKGROUND

Technical Field

The present embodiments relate to thin film solar cells. Morespecifically, the embodiments relate to a stacked-layered thin filmsolar cell with improved performance characteristics.

Description of the Prior Art

The art of solar cells addresses the conversion of radiation intoelectrical energy. Much research has been conducted to maximize theefficiency of a solar cell. There is a demand to further reduce theabsorber thickness of a solar cell to reduce material costs, decreasethe time required for deposition processing, and increase the throughputof deposition equipment. Shortcomings however, include the back contactof a solar cell having reduced reflectance for infrared wavelengths anda surface with a high probability for charge carrier recombinationleading to decreased efficiency of the solar cell.

SUMMARY

The embodiments disclosed herein comprises a process for preparing athin-film solar cell.

In one aspect, the process employs etching one or more apertures,filling at least one aperture with a material, and depositing atransparent layer. A first aperture is etched into a second side of areflective layer and a second aperture into a second side of adielectric layer. The first and second apertures form a third aperture.A first side of the dielectric layer is in contact with a body, with thebody comprising a substrate having a first side and a second side, and afirst conducting material layer having a first side and a second side,the first side of the first conducting material in contact with thesecond side of the substrate. The third aperture is filled with a layerof Copper Indium Gallium Selenide (CIGS) having a first side and asecond side. The first side of the CIGS is in contact with the firstconductive layer of the dielectric within the third aperture and coversthe second side of the dielectric layer and the second side of the CIGSin contact with a first side of a first transparent layer. A secondtransparent layer is deposited with a first side and a second side onthe first transparent layer. This depositing includes the first side ofsecond transparent layer in contact with a second side of the firsttransparent layer. The second transparent layer is a conductingmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referenced herein form a part of the specification.Features shown in the drawings are meant as illustrative of only someembodiments, and not of all embodiments unless otherwise explicitlyindicated. Implications to the contrary are otherwise not to be made.

FIG. 1 depicts a flow chart illustrating a process for creating animproved solar cell.

FIG. 2 is a block diagram depicting an improved solar cell.

FIGS. 3A and 3B are illustrative drawings depicting soft stamping andetching a solar cell.

DETAILED DESCRIPTION

It will be readily understood that the components of the presentembodiments, as generally described and illustrated in the Figuresherein, may be arranged and designed in a wide variety of differentconfigurations. Thus, the following detailed description of the presentembodiments of the apparatus, system, and method, as presented in theFigures, is not intended to limit the scope of the embodiments, asclaimed, but is merely representative of selected embodiments.

Reference throughout this specification to “a select embodiment,” “oneembodiment,” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. Thus, appearances of the phrases“a select embodiment,” “in one embodiment,” or “in an embodiment” invarious places throughout this specification are not necessarilyreferring to the same embodiment.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided, such asexamples of sensors, detectors, etc., to provide a thoroughunderstanding of embodiments. One skilled in the relevant art willrecognize, however, that the embodiments can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the embodiments.

The illustrated embodiments will be best understood by reference to thedrawings, wherein like parts are designated by like numerals throughout.The following description is intended only by way of example, and simplyillustrates certain selected embodiments of devices, systems, andprocesses that are consistent with the embodiment(s) as claimed herein.

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and which shows byway of illustration the specific embodiment(s) which may be practiced.It is to be understood that other embodiments may be utilized becausestructural changes may be made without departing form the scope of thepresent embodiments.

FIG. 1 depicts a flow chart (100) illustrating a method for manufactureof a stacked-layered thin film solar cell. A conducting layer isdeposited on a substrate such that a second side of the substrate is incommunication with a first side of a conducting layer (102). Areflective layer is deposited on the conducting layer such that a firstsurface of the reflective layer is in communication with a second sideof the conducting layer (104). The second side of the conducting layeris oppositely disposed from the first side of the conducting layer. Adielectric layer is deposited on the reflective layer. Specifically, afirst side of the dielectric layer is deposited such that it is incommunication with a second side of the reflecting layer, which isoppositely disposed from the first side of the reflective layer (106). Asecond side of the dielectric layer, which is oppositely disposed to thefirst side of the dielectric layer, is coated with a polymer (108). Thepolymer is structured (110), and in one embodiment, is cured by exposingthe polymer to ultraviolet radiation (112). At least one aperture iscreated within both the dielectric layer and the reflective layer (114).The aperture is formed to create electrical contact between theconducting layer and the semiconducting layer. In one embodiment, theaperture is formed by etching through the dielectric layer and into thelayer comprised of a reflective material. Accordingly, an insulatingdielectric layer is in between the semiconducting material and theconducting material, and the aperture through the reflective layer isemployed for ohmic contact.

A layer of Copper Indium Gallium Selenide (CIGS) is deposited on thesecond side of the dielectric layer (116). The CIGS functions as ap-type semiconductor. The CIGS layer is deposited such that a first sideof the CIGS is in physical contact with the second side of thedielectric and fills at least one of the apertures formed in thedielectric and reflective layers. In one embodiment, the CIGS isdeposited on the second side of the dielectric layer (116) byco-evaporating copper, gallium, indium, and in one embodiment, selenium,on the dielectric (116). A first transparent layer is deposited on theCIGS layer such that a first side of the first transparent layer is incommunication with a second side of the CIGS layer, oppositely disposedfrom the first side of the CIGS layer (118), including Cadmium (Cd),Sulfur (S), and/or Zinc Oxide (ZnO), or any other transparent materialthat readily forms a charge carrier separating junction with the CIGSlayer. A conducting second transparent layer is deposited on the firsttransparent layer such that a first side of the conducting secondtransparent layer is in communication with a second side of the firsttransparent layer, oppositely disposed from the first side of the firsttransparent layer (120). In one embodiment, the conducting layer and thereflecting layer are deposited through the use of sputtering. Ananti-reflective coating is applied to a second side of the conductingsecond transparent layer (122). The second side of the conducting secondtransparent layer is oppositely disposed to the first side of theconducting second transparent layer. The anti-reflective coatingdecreases the reflectance of radiation contacting the conducting secondtransparent layer, allowing for less reflection and hence greaterabsorption of radiation and thus an increased efficiency of the solarcell. In one embodiment, areas of the solar cell are separated andinterconnected into sub cells using mechanical or laser-scribing (124)in order to increase module voltage and reduce ohmic losses within theinterconnections. In another embodiment, the stacked-layered thin filmsolar cell as described above is encapsulated to form a module (126) toprotect the layers from mechanical and environmental degradation, suchas corrosion. Accordingly, a stacked-layered thin film solar cell ismanufactured by the method as described.

FIG. 2 is a block diagram illustrating a stacked-layered thin-film solarcell. The solar cell is shown with a substrate layer (202) and a layerof a conducting material (204). The substrate is shown with a firstsurface (222) and a second surface (232). Similarly, the layer ofconducting material (204) is shown having a first surface (224) and asecond surface (234), with surface (224) stacked on the substrate layer(202). In one embodiment, the substrate is constructed from soda limeglass, solar glass, aluminum foil, iron (Fe), and/or polyimide (PI). Theconducting material (204) provides ohmic contact to the semiconductorabsorber (214) and lowers resistance for in-plane current flow. In oneembodiment, the conducting material (204) is constructed from Molybdenum(Mo). The solar cell further includes a layer having a reflectingelement (206) with a first surface (226) and a second surface (236), thereflecting element (206) stacked on the conducting material (204). Inone embodiment, the reflecting element (206) is silver. A dielectriclayer (208) is shown in communication with the reflecting layer (206).The dielectric layer (208) has a first surface (228) and a secondsurface (238), the first surface (228) is stacked on the reflectinglayer (206). In one embodiment, the reflecting layer (206) includes anelement reflective of solar radiation to reflect contacting radiationinto the absorber layer (214).

At least one aperture is etched through the dielectric layer (208) andinto the reflecting layer (206). While two apertures, (210) and (212)respectively, are depicted in FIG. 2, any number of apertures can beetched through the dielectric layer (208) and into the reflecting layer(206). In one embodiment, the thickness of the aperture(s) ranges from0.1 to 0.15 micrometers. A layer of a semiconducting material (214),having a first surface (244) and a second surface (254), is stacked onthe dielectric layer (208). In one embodiment, the semiconductingmaterial is CIGS. The semiconducting layer (208) provides for electricalcharacteristics at the surface (244) of the semiconducting layer (214)in contact with the dielectric layer (208).

The semiconducting layer (214) fills the aperture(s), (210) and (212)respectively, formed within the dielectric layer (208) and reflectinglayer (206). A first transparent layer (216) is provided as a layerwithin the solar cell and in communication with the semiconducting layer(214). In one embodiment, the first transparent layer (216) is comprisedof Zinc Oxide (ZnO). The first transparent layer (216) has twooppositely disposed surfaces (246) and (256). In one embodiment, thesemiconducting layer (214) has a base thickness between 0.5 and 5micrometers. The base thickness is considered to be the shortestdistance between the surface of the dielectric (238) in contact with thesurface (244) of the semiconducting layer (214) to the surface (254) ofthe semiconducting layer (214) in contact with the surface (246) of thefirst transparent layer (216). The base thickness of the semiconductinglayer therefore discounts the semiconducting material filling theaperture(s) in the dielectric and reflecting layers, (208) and (206),respectively.

A conducting second transparent layer (218) is provided. In oneembodiment, the conducting second transparent layer is comprised of ZincOxide doped with Aluminum (ZnO:Al). This layer (218) has two oppositelydisposed surfaces (248) and (258) having a first surface (248) and asecond surface (258) stacked on the first transparent layer (216). Theconducting second transparent layer (218) collects current from absorbedradiation. In one embodiment, the solar cell is placed in communicationwith a contact grid to collect electrical current. Accordingly, astack-layered solar cell is provided.

FIG. 3A is a block diagram (300) illustrating one embodiment for a softstamp to etch the dielectric and silver layers of the stack-layeredsolar cell of FIG. 2. A substrate layer (302) is provided with aconducting layer (304) stacked on the substrate layer (302). A layercomprising a reflecting element (306) is further provided stacked on theconducting layer (304), and a dielectric layer (308) is provided stackedon the reflecting layer (306). A polymer layer (310) coats thedielectric layer (308). A soft stamp (320) is shown in communicationwith the solar cell layers to provide an etch mask. In one embodiment,the stamp (320) is a Poly-dimethylsilxane (PDMS) stamp. Similarly, inone embodiment, the polymer is removed from the protruded areas of thestamp, (350) and (352) respectively, and the remaining polymer isexposed. In one embodiment the ultraviolet radiation may be employed tocure the remaining polymer. In another embodiment, the protruded areas(350) and (352), respectively, of the soft stamp (320) are separated bya distance commensurate with an optimized electrical performance of thecell. Accordingly, the soft stamp structures the polymer to provide anetch mask subsequent to etching.

FIG. 3B depicts a block diagram illustrating the substrate layer (302),the conducting layer (304), the reflecting layer (306) and thedielectric layer (308), subsequent to etching. Two etched apertures,(312) and (314) formed by the stamp are depicted. In one embodiment, thedielectric and the reflecting elements are etched through such that atleast one aperture is formed. At least one aperture is etched throughthe entire dielectric layer and stops within the reflecting layer (310).Similarly, in one embodiment, at least one aperture (312) is etchedcompletely through the dielectric (308) and the reflecting element(306). Accordingly, aperture(s) (312) and (314) are formed by etchingthrough the dielectric and reflective layers.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises” and/or “comprising,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present embodiments has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the form disclosed.

Many modifications and variations will be apparent to those of ordinaryskill in the art without departing from the scope and spirit of theembodiments. The embodiment was chosen and described in order to bestexplain the principles and the practical application, and to enableothers of ordinary skill in the art to understand the variousembodiments with various modifications as are suited to the particularuse contemplated.

It will be appreciated that, although specific embodiments have beendescribed herein for purposes of illustration, various modifications maybe made without departing from the spirit and scope. Accordingly, thescope of protection is limited only by the following claims and theirequivalents.

We claim:
 1. A solar cell, prepared by a process comprising the stepsof: etching a first aperture through a reflective layer and a secondaperture through a dielectric layer, the first and second apertures toform a third aperture, wherein a first side of the reflective layer isin contact with a second side of a conducting layer, a first side of thedielectric layer is in contact with a second side of the reflectivelayer, a first side of the conducting layer is in contact with a firstside of a substrate; filling the third aperture with a layer of CopperIndium Gallium Selenide (CIGS) having a first side and a second side,the first side of the layer of CIGS is in direct physical contact withthe second side of the conducting layer, the second side of theconducting layer continuously extends across the filled third apertureand covering a second side of the dielectric layer and the second sideof the CIGS in contact with a first side of a first transparent layer;and depositing a second transparent layer, including a first side ofsecond transparent layer in contact with a second side of the firsttransparent layer, wherein the second transparent layer comprises aconductive material.
 2. The solar cell of claim 1, further comprisingcoating the second side of the dielectric layer with a UV-curablepolymer prior to soft stamping and etching the first and secondaperture, and removing the UV-curable polymer subsequent to the softstamping and etching.
 3. The solar cell of claim 2, further comprisingcuring the UV-curable polymer by exposing the UV-curable polymer toultraviolet light.
 4. The solar cell of claim 1, wherein the conductinglayer is prepared by sputtering.
 5. The solar cell of claim 1, whereinthe reflective layer is prepared by sputtering.
 6. The solar cell ofclaim 5, further comprising separating and interconnecting areas of thesolar cell into sub cells by a method selected from the group consistingof mechanical scribing and laser-scribing.
 7. The solar cell of claim 1,further comprising encapsulating the solar cell to form a module.
 8. Thesolar cell of claim 1, wherein the conducting layer comprisesMolybdenum.
 9. A solar cell, prepared by a process comprising the stepsof: depositing a conducting layer on a substrate layer, wherein a firstsurface of the substrate layer is in communication with a first surfaceof the conducting layer; depositing a reflective layer on the conductinglayer, wherein a second surface of the conducting layer is incommunication with a first surface of the reflective layer; depositing adielectric layer on the reflective layer, wherein a second surface ofthe reflective layer is in communication with a first surface of thedielectric layer; etching one or more apertures, wherein the one or moreapertures extend through the dielectric layer and the reflective layer;depositing a semiconductor layer, including: filling the one or moreapertures with Copper Indium Gallium Selenide (CIGS), wherein a firstportion of a first surface of the semiconductor layer is in directphysical communication with the second surface of the conducting layerand the second surface of the conducting layer continuously extendsacross the filled one or more apertures; covering a second surface ofthe dielectric layer with the CIGS, wherein a second portion of thefirst surface of the semiconductor layer is in communication with thesecond surface of the dielectric layer; depositing a first transparentlayer on the semiconductor layer, wherein a first surface of the firsttransparent layer is in communication with a second surface of thesemiconductor layer; and depositing a second transparent layer on thefirst transparent layer, wherein a first surface of second transparentlayer in communication with a second surface of the first transparentlayer, the second transparent layer comprises a conductive material. 10.The solar cell of claim 9, further comprising coating the second surfaceof the dielectric layer with a UV-curable polymer prior to a softstamping and etching the one or more apertures, and removing theUV-curable polymer subsequent to the soft stamping and etching.
 11. Thesolar cell of claim 10, further comprising curing the UV-curable polymerby exposing the UV-curable polymer to ultraviolet light.
 12. The solarcell of claim 9, wherein the depositing the conducting layer includessputtering.
 13. The solar cell of claim 9, wherein depositing thereflective layer includes sputtering.
 14. The solar cell of claim 13,further comprising separating areas of the solar cell into sub cells bya method selected from the group consisting of mechanical scribing andlaser-scribing.
 15. The solar cell of claim 14, further comprisinginterconnecting the sub cells of the solar cell.
 16. The solar cell ofclaim 9, further comprising encapsulating the solar cell to form amodule, wherein the encapsulating prevents mechanical and environmentaldegradation.
 17. The solar cell of claim 9, wherein the conducting layercomprises Molybdenum.
 18. A solar cell, prepared by a process comprisingthe steps of: depositing a conducting layer on a substrate layer,wherein a first surface of the substrate layer is in communication witha first surface of the conducting layer; depositing a reflective layeron the conducting layer, wherein a second surface of the conductinglayer is in communication with a first surface of the reflective layer,wherein a first aperture is formed in the reflective layer; depositing adielectric layer on the reflective layer, wherein a second surface ofthe reflective layer is in communication with a first surface of thedielectric layer, wherein a second aperture is created in the dielectriclayer, wherein the first and second apertures are created in alignmentforming a third aperture; depositing a semiconductor layer, including:filling the third aperture with Copper Indium Gallium Selenide (CIGS),wherein a first portion of a first surface of the semiconductor layer isin direct physical communication with the second surface of theconducting layer; covering a second surface of the dielectric layer withthe CIGS, wherein a second portion of the first surface of thesemiconductor layer is in communication with the second surface of thedielectric layer; depositing a first transparent layer on thesemiconductor layer, wherein a first surface of the first transparentlayer is in communication with a second surface of the semiconductorlayer; and depositing a second transparent layer on the firsttransparent layer, wherein a first surface of second transparent layerin communication with a second surface of the first transparent layer,the second transparent layer comprises a conductive material.